Natural gas production can be undesirable in wellbores in many oil and gas fields, such as those of the North Slope of Alaska. Such natural gas production can be particularly problematic in horizontal wells. Even when a horizontal wellbore is located a hundred feet below the gas-oil contact (GOC), undesirable gas production often still occurs. Possible sources of such gas production include channeling through a casing/borehole annulus when there is poor cement isolation, coning through higher permeability sections of the reservoir, suction of gas through permeable fault planes, or some combination thereof.
Production log flow profiles provide a valuable tool to evaluate well and reservoir performance, and to identify and eliminate undesirable gas production. Beginning with an analysis of the well's initial completion efficiency, downhole flow profiles provide zonal production allocation and analysis of break-through and coning events and are also used to check the mechanical integrity of completions. Production log surveys are commonly run to diagnose problems when anomalies in surface production rates are noticed. In such cases, the survey results are used to plan remedial actions to eliminate gas production and bring the well back to its optimum producing efficiency.
Production log flow profiles are generated using a conventional production logging tool string which typically includes spinner, capacitance, fluid density, temperature, and pressure sensors. The spinner is used to measure the total velocity of the fluid mixture; the capacitance and density sensors are used to determine the instantaneous holdups of water, oil, and gas; and the temperature sensor is used to identify fluid entries. To correctly interpret a flow profile from these sensors, the measured velocity must be converted to flow rate and individual velocities for each phase must be determined. While correlations for these interpretations are well understood in vertical wells, such correlations become less reliable as the angle of the well trajectory from vertical increases. Horizontal wells, thus, represent an extreme challenge for conventional production log interpretation.
The foregoing problems with interpreting production log flow profiles are further complicated by phase segregation, which can cause extreme fluctuations in spinner response, and minor trajectory deviations which generate widely changing individual phase velocities. Additionally, in horizontal wells having slotted liners, fluids can flow both inside the slotted liner and in the annulus between the liner and the openhole, resulting in variations in fluid velocity as the fluid enters and exits the slotted liner. Still further, restrictions inside the slotted liner can also cause erroneously high fluid velocities. Because the fluid velocity may vary as the fluid flows through the liner and/or the annulus rather than from the reservoir, the velocity sensed by the spinner may or may not reflect fluid flow from the reservoir, resulting in erroneous spinner counts and, consequently, erroneous flow rate interpretations.
In addition to conventional production logs, pulsed neutron capture (PNC) logs have been used to monitor gas, to detect cement channeling, and to locate gas and water entry by measuring thermal neutron capture cross-sections, such as sigma (.SIGMA.) formation, sigma (.SIGMA.) borehole, and porosity (e.g., thermal PNC porosity, or "TPHI"). In cement channel detection, for example, a PNC log is used to measure the formation sigma through a wellbore as a base pass. Then a high-capture cross-section solution, such as a saturate sodium borate solution, i.e. borax, is injected into the wellbore as a tracer to detect cement channels, and the sigma of the wellbore is measured again. Because the capture cross-section of borax is much higher than the capture cross-section of fresh water or oil, it produces a strong signature in the sigma measurements where the borax solution is injected. The sigma measurements made before and after the injection of the borax are overlaid, and the difference, or separation, between the measurements is used to detect the tracer in the cement channels. While the presence of separation is used as an indication of cement channels, the magnitude of the separation has not been evaluated.
While conventional PNC logs provide a measure of the porosity and sigma of a formation, they do not provide a direct measure of the permeability of a formation which is necessary to assess the flow profile and potential productivity of a wellbore.
Therefore, what is needed is a method for generating a flow profile of a wellbore from pulsed neutron logs which may be reliably interpreted to determine the flow profile of a wellbore, particularly of a horizontal wellbore having a slotted liner completion, and to determine from the flow profile, the flow rate and individual velocities for each phase of fluid flowing through the wellbore.